1. Field of the Invention
The present invention relates generally to a spark ignition timing control system for an internal combustion engine, such as an automotive internal combustion engine. More specifically, the invention relates to a spark ignition timing control system with a fail safe system for a cylinder pressure sensor which monitors fluctuation in pressure within the combustion chamber of an engine cylinder of the internal combustion engine. Further particularly, the invention relates to a fail safe system for a cylinder pressure sensor used for detecting and controlling knocking in the internal combustion engine, on the basis of the fluctuation in pressure within the combustion chamber.
2. Description of the Prior Art
In order to monitor fluctuation in pressure within a combustion chamber of an engine cylinder, which will be referred to as "cylinder pressure", a cylinder pressure sensor made of a piezoelectric ceramic has been used. A signal produced by the cylinder pressure sensor is processed by means of a band-pass filter after performing its charge/voltage conversion, and then rectified. Thereafter, the rectified signal is integrated within a predetermined range every combustion cycle of each of the engine cylinders, so that a physical quantity S for combustion energy within a combustion chamber of the engine cylinder every combustion cycle can be obtained. Therefore, after a running average S of the obtained physical quantity S is calculated, knocking in the internal combustion engine can be detected by comparing a deviation S-S with a predetermined value SL. That is, when the deviation S-S is greater than the value SL, it can be assumed that knocking is occuring in the internal combustion engine, in which case the spark ignition timing is so controlled as to be retarded. On the other hand, when the deviation S-S is smaller than the value SL, it can be assumed that no knocking is occuring in the internal combustion engine, and the spark ignition timing is so controlled as to be advanced.
In the case of controlling the spark ignition timing on the basis of detected knocking, it is required for reliable data to be outputted from the cylinder pressure sensor. Therefore, when the cylinder pressure sensor is abnormal, or malfunction occurs in the cylinder pressure sensor, it is required for this to be detected.
Knocking in the engine cylinder is a proability phenomenon, and the aforementioned physical quantity S represents a predetermined dispersion in normal conditions. From this point of view, the Japanese Patent First (unexamined) Publication Showa No. 62-34028 discloses a spark ignition timing control system in which whether or not the cylinder pressure sensor is abnormal is determined, and the spark ignition timing is appropriately corrected if the sensor is abnormal.
In the aforementioned system, a statistical dispersion (S-S).sup.2 of the physical quantity S is calculated, and then a running average Z of this dispersion is calculated by, for example, the following equation, each combustion cycle. EQU Z=(15/16)Z+(1/16)(S-S).sup.2
Thereafter, under specific sets of driving conditions, e.g. high load or high engine speed and high load, the running average of the dispersion Z is compared with a predetermined value. When the running average Z is greater than the predetermined value, it is assumed that normal combustion is occuring in the engine cylinder. On the other hand, when the running average Z is smaller than the predetermined value, it is assumed that abnormal combustion is occuring therein, and fail safe processing, e.g. retardation of the spark ignition timing, is performed. For example, under low load conditions, little knocking occurs, so that the physical quantity S is very small, therefore the dispersion Z is small. Therefore, the conditions under which determination is made as to whether or not combustion in the engine cylinder is abnormal, are limited to the aforementioned specific sets of conditions.
However, in such a conventional knocking detection system, under the specific driving conditions, wherein in functioning condition of the sensor is detected, the running average is obtained by continuously updating the running average of the dispersion. Therefore, during a short period immediately after a driving conditions changes from one having a small dispersion into one of the specific driving conditions, the calculated running average is small.
Under certain running conditions, it is assumed that a certain threshold dispersion must be encountered. If that threshold value is not reached, then it is assumed that the sensor is broken. Therefore, when the running condition of the engine changes from one having a very low dispersion to one having a high dispersion, the running avarage remains low for a short period. SInce the running average of the dispersion appears to be too low for the running condition of the engine, the CPU of the system erroneously assumes that the sensor has broken and goes to fail safe mode.